This invention relates to proximity detection apparatus, centre-finding apparatus, coordinate measuring apparatus, surface levelling apparatus, and thickness measurement apparatus employing changes in reactance responsive to the physical quantity measured.
When a machine such as a lathe or milling machine is to be used to accurately perform a function relative to a position on a workpiece, some reference point on the workpiece must be used to align the machine. When the reference point is the centre of a bore or the centre of a shaft, measurement techniques must be employed to accurately determine the centre's position relative to the machine's coordinate space.
Proximity detection probes can be used to determine the contour of the bore or shaft, however, complex mathematical calculations are often required to calculate the centre and often the result of the calculation is subject to precision limitations. Even if a centre is calculated, there may still remain the problem of aligning a tool holder or other object with the centre, which is further subject to adjustment errors. Effectively, these methods of alignment should be considered "open loop methods" as no feedback is continuously provided during the alignment process.
Proximity detection devices employing changes in capacitance created between a probe tip and a workpiece are known and described in U.S. Pat. No. 5,189,377 to Rhoades et al. Such devices include circuitry which measures capacitance between the probe tip and the workpiece and provide a signal operable to control a numerically controlled machine. The circuitry is typically connected to the machine by wires which limit movement of the device such that it must remain stationary or at least non-rotational relative to the machine.
U.S. Pat. No. 4,670,989 to Juengel discloses a touch probe which transmits a microwave signal modulated by frequency shift keying in response to contact of the probe with an object. Discrete, rather than continuous proximity information is provided by this probe and therefore it is unable to present information relating to the distance of the probe to the workpiece. Such information must be provided in conjunction with other equipment.
Effectively, the Rhoades et al. device is capable of providing continuous positional information however it is tethered to the machine on which it is installed. The Juengel device is not tethered but does not provide continuous proximity information. Neither of these probes can be rotated while at the same time providing continuous proximity information which limits or excludes their use in centre finding applications.
U.S. Pat. No. 4,118,871 to Kirkham discloses a radio frequency touch probe which indicates a workpiece contact condition through changing the radiation characteristics of the probe tip/antenna. A substantial drop in output signal indicates a contact condition. This relies on establishing a low impedance contact, at the transmitted frequency (not DC), between the probe tip/antenna and workpiece. A reliable low impedance contact is not always present due to oxidation and the like. Additionally, the Kirkham device does not provide simultaneous proximity information while probing which results in a "blind" probing system requiring low probing speeds and correction factors to account for probe tip bending and positional overshoot of the machine system.
The patent of Kirkham relies solely on a low impedance contact to effectively "short out" the radiating probe tip to indicate the contact condition. It therefore works by changing the radiation characteristics of the radiator. Also, the Kirkham probe, apart from not providing simultaneous proximity capability with a single probe tip, would not work effectively when the probing operation effectively shields the primary tip radiator, such as would be the case when probing a deep bore. It would be desirable therefore to have a device which can transmit continuous proximity information when rotating or stationary.